To prevent as much as possible the incursion of water upon the re claimed land, as well as to reduce the volume there accumulated, the water courses traversing the territory are as far as practicable intercepted and made to pass around the lowlands by diverting them—often at considerable expense—into new channels.
plan most commonly followed for lower ing the water-level of a locality consists in establishing a network of canals so disposed that the smaller shall open into the larger, these, in turn, into the creeks and lesser streams, and these into the rivers, thus affording a connected system of outlet channels. This system can be applied only where the available fall is sufficient for carrying it into effect. That the lateral channels opening into it may be given the greatest possible fall, the main drainage-channel follows the line of the deepest depression in the marsh. As the size of the drainage-channels will depend on the quan tity of water to be drained, they must be cut deep enough to obtain the required lowering of the water-level over the entire drained area. For a meadow the water-level should be about 15 inches below the g-round-sur face, and for cultivated land about 4 feet.
Covered-drainage 5),stons.—Itistead of the above-described open-drain ag-e canals, covered channels or drains are frequently employed. This plan is especially adapted for clayey soils devoted to agricultural uses, for in such cases the distance between the canals must necessarily be small, and open channels, aside from other objections, would involve the loss of much land valuable for agricultural uses. These covered drains—which are placed at a suitable depth and have the necessary fall—constitute a series of hollow spaces in which the soil-water collects and is filially discharged into ditches with which they communicate. For this purpose it is cus tomary to use tubes of baked clay a few inches in diameter, made in sec tions from foot to Y; feet in length, which, being laid end to end in trenches of proper depth and suitable width, are covered with earth. The ends of these tubes are fitted together with a collar which holds them snugly enough in place, but still permits the entrance of water by percola tion throng-1i the joints. As the surrounding soil is more difficult of pene tration than the openings about the joints of the tubes, the soil-water col lects in these, and is conveyed to the drainage-canal provided for it. The
depth at which this underdraining should be located will depend upon the nature of the ground and the kind of cultivation to which it is to be sub jected. The depth should not be less than 2 feet. The small drains just described (suction-drains) conmi tin icate with larger ones (collecting-drains); the former follow the direction of the g-reatest fall. This should be at least .o4 per cent. for tubes of one inch diameter, and the distance apart should be from Io to 4o feet. In place of the above-described tubes of baked clay, hollow tiles are employed for the same purpose, and likewise canals filled with small stones; in firm clayey soil simple hollows, left unfilled, have been used, but no reliance can be placed upon their permanence.
Artificial Drainagc.—If the water-surface lies too low to permit of drainage by utilizing a natural outlet, recourse must be had to appliances for artificially raising- the water. Of these, the most commonly nsed are bucket-wheels, Archimedean screws, and especially pumps (centrifugal pumps, among others). The motive-power for operating these machines is to some extent still furnished by the wind, by water-power, or by ani mals. For important works of this nature, however, powerful steam engines are employed. One of the most extensive operations of this kind is the drainage of the Haarlem Lake, in Holland, which covered an area of seventy-two square miles. To effect this, the nicer was first isolated by constructing- about it a strong circular dyke, around which was built a canal thirty-nine miles in length. This communicated with Lake V, and with all the canals which conduct the water of the ineer to the North Sea. Three pumping-engines of four hundred horse-power each raise the water from the lake, and two others lift the water out of the surrounding- canal into Lake where, on account of the tides, the level of the sea outside is higher than that of the canal. Nine hundred and forty-six million cubic yards of water were pumped out in somewhat more than three years. The machinery is now required merely to prevent the further accumulation of water, and is operated during only a few months of the year.